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I was having a discussion with a friend of mine about fiber and he said that he has GBICs that support both Single-mode and Multi-mode fiber in the same device. At first I thought, no way, that can't be, that doesn't make any sense.

My thinking was this:

  1. Single-mode uses a laser while Multi-mode uses an LED to generate the signal. This would require two significantly different devices to generate the signal.

  2. The core sizes are drastically different between Single-mode and Multi-mode. SM being 9 micron and Multi-mode 62.5 or 50 micron.

  3. Wouldn't the photo receiver (or whatever they use) be very different and have trouble dealing with the two different types of signal.

  4. It doesn't really make much sense from an administration stand point to me. I mean I understand the idea of plugging something in and having it just work no matter what, but anyone who is plugging in this stuff should probably know the difference and how to identify which fiber is what type. Wouldn't it be more cost prohibitive to jam these two different technologies into one small GBIC?

So, I looked it up and according to CISCO the Cisco GLC LH SM SFP GBIC is an example of one that "is able to work with both single-mode and multi-mode fiber". So I suppose I am wrong.

My question is, is this true, has anyone used one of these MM/SM GBICs? When did they start making these and why did they start making these?

renosis
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2 Answers2

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The particular GBIC you're referring to requires the use of a mode-conditioning patch cord when connected to a multi-mode fiber cable. The GBIC doesn't really "support" multi-mode fiber, per se, but rather this patch cord provides the necessary optical "distortion" to make the modulation work properly over a multi-mode fiber.

Evan Anderson
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  • This is only necessary for multi-mode runs greater than [300 meters](http://www.cisco.com/en/US/docs/routers/7200/install_and_upgrade/gbic_sfp_modules_install/5067g.html#wp34206). – James Sneeringer Mar 06 '13 at 22:51
  • This is true. My understanding is that this is necessary to overcome the optical differences in multi-mode versus single-mode which begin to have an effect on the signal in longer runs. – Evan Anderson Mar 06 '13 at 22:56
  • Likely. I inherited a number of runs just like this at my current job. All have worked flawlessly during my time here, but none are even 100 meters, let alone 300. – James Sneeringer Mar 06 '13 at 22:58
  • I see. Thanks Evan Anderson. I think I would stick to buying specifically MM for MM fiber and SM for SM fiber. I always knew you could force SM through MM or vice-versa on really short distances. I was kind of shocked when I saw this GBIC saying it supports both MM and SM. – renosis Mar 07 '13 at 18:28
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Perhaps a better way to think of it is: SX - used with multimode LX - used with either multimode or singlemode, operating distance determined by which you use.

Tom Mikelson
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